Wide angle optical retarder

ABSTRACT

In an acrylonitrile based retarder, relatively uniform performance is obtained as the angle of incidence varies over a wide range. A toughening material may be blended with an acrylonitrile based polymer to facilitate processing of a retarder and improve mechanical properties of the retarder without compromising the optical performance. A rubber-modified acrylonitrile based retarder which provides relatively uniform wide angle can be fabricated using conventional processing techniques. Such retarders are particularly suited for a number of specific applications which use relatively wide ranges of incidence angles.

This is a continuation of application Ser. No. 08/953,128 filed Oct. 17,1997 now U.S. Pat. No. 5,867,239.

BACKGROUND

This invention relates generally to an optical retarder, and moreparticularly to an optical retarder operable over a wide range ofincidence angles.

Optical retarders are generally used in some manner to alter therelative phase of polarized light passing through the retarder. Opticalretarders are particularly suited for use in applications where controlover the polarization is required. Polarization of light generallyrefers to the restriction of electric (or magnetic) field vectorvibrations to a single plane. The polarization direction ofelectromagnetic radiation is generally considered the direction in whichthe electric field vector oscillates back and forth. The polarizationvector is orthogonal to the beam direction within the light plane.

Polarized light can assume a number of different forms. Where a lightbeam oscillates in only one direction at a given point the beam is saidto be linearly (or plane) polarized. The direction of oscillation is thepolarization direction. If the light beam has two orthogonalpolarization directions which vary in phase by 90°, the beam is said tobe elliptical or circularly polarized. Circular polarization occurs whenthe magnitude of the two oscillations are equal (i.e., the tip of theelectric field vector moves in a circle). Elliptical polarization occurswhen the magnitudes are not equal (i.e., the tip of the electric fieldvector moves in a ellipse). In contrast, the orthogonal oscillations forunpolarized light are on average equal with a randomly varying phasedifference.

Linearly polarized light can be obtained by removing all waves from anunpolarized light beam except those whose electric field oscillate in asingle plane. Optical retarders can be used, for example, to convertlinearly polarized light to circular or elliptically polarized light.When used to control the polarization of light, retarders are commonlyconstructed to induce ½- and ¼-wave retardations. Generally, suchretarders are used to produce a desired relative phase delay between twolinear-components of the polarized light.

One typical use of an optical retarder is a compensator which is used tointroduce a phase delay in incident light to correct for phasedifferences between two components of polarized light introduced bymechanical or optical displacement of other optical components in asystem. In a liquid crystal display (LCD), for example, birefringence ofa liquid crystal cell may cause the linearly polarized light to becomeslightly elliptical. A retarder is used to convert the ellipticallypolarized light back to linearly polarized light. The compensatingretarder is placed in the light path and is tuned to a particular phasedifference introduced by the birefringence of the liquid crystal.

Typical optical retarders are constructed of birefringent materials. Thebirefringent materials form a fast and slow path along two orthogonalin-plane axes of the retarder. When the axes of the birefringentretarder are aligned at 45° degrees to the polarization direction of theincident light, the retarder can be used to introduce or compensate forphase differences between two polarization components. The fast and slowpath of the birefringent retarder results from different refractiveindices for light polarized along the in-plane axes of the retarder.Larger retardation differences between the two polarization axes areachieved by increasing the refractive indices difference between the twoin-plane axes and/or increasing the thickness of the retarder. Thus, bycontrolling the thickness and refractive indices of the birefringentmaterial in the retarder, the optical properties of the retarder can becontrolled.

In addition to refractive indices for light polarized along the in-planeaxes of the retarder, the refractive index for light polarized in thethickness direction may influence the performance of the retarder in agiven application. Compensators used in LCD display technology, forexample, must provide relatively uniform retardation of light which isincident on the compensator over a relatively large angle range. It hasbeen proposed that widened viewing angle ranges for LCD displays areobtainable by employing retardation films which have controlledrefractive indices for light polarized in the thickness direction.

Current attempts to produce retarders having uniform wide angleperformance have proven to be expensive and difficult to manufacture andhave only achieved limited success in obtaining uniform wide angleoptical properties. Attempts to obtain uniform wide angle performanceare varied and include, for example, shrinking the film in the directionperpendicular to the stretching direction at the time of stretching,controlling, by stretching, the birefringence of a raw film producedfrom a molten polymer or a polymer solution under an applied electricfield, laminating a film produced under an electric field onto aconventional phase retarder, and the like. Such processes are typicallyquite complex and expensive and achieve only limited success. As theprocess and materials used in forming the birefringent portion of aretarder become more complex, it becomes increasingly difficult toincorporate such material into the retarder.

SUMMARY

Generally, the present invention relates to optical retarders. In oneembodiment, an optical retarder is provided which uniformly retardslight incident on the retarder over a relatively wide range of incidenceangles varying from an angle normal to a plane of the retarder to amaximum angle of at least about 30 degrees. The optical retarder caninclude a substrate and a blended film of an acrylonitrile based polymerand elastomeric copolymer disposed on the substrate. The magnitude ofretardation varies by less than about 25% of the normal angle incidenceretardation as the angle of incidence varies from normal incidence toincidence at the maximum angle. In one embodiment the maximum angle maybe greater than about 60 degrees. When the maximum angle is smaller thevariance in retardation may be less.

In another embodiment, an acrylonitrile based retarder mirror isprovided. Linearly polarized light reflected by the retarder mirror isrotated to a substantially orthogonal linear polarization. The rotationof the polarization orientation is relatively uniform over a relativelywide range of incidence angles onto the retarder mirror. In anotherembodiment, an anti-reflective optical construct includes anacrylonitrile based retarder to improve off-normal angular performanceof the anti-reflective construct.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIGS. 1A-1B illustrate a retarder in accordance with one embodiment ofthe invention;

FIGS. 2A-2C illustrate characteristics of a retarder in accordance withan embodiment of the invention;

FIGS. 3A-3C illustrate characteristics of a retarder in accordance withan embodiment of the invention;

FIGS. 4A-4B illustrate characteristics of a retarder in accordance withan embodiment of the invention;

FIGS. 5A-5B illustrate a particular application of an optical retarderin accordance with one embodiment of the invention;

FIG. 6 illustrates another particular application of an optical retarderin accordance with one embodiment of the invention; and

FIG. 7 illustrates still another particular application of an opticalretarder in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

The present invention is applicable to a number of optical retarders.The present invention is particularly suited to optical retarders usedin environments where the light to be retarded is incident on theretarder over a relatively wide range of incidence angles. Such aretarder is well suited for use as optical compensators, ½-wave and¼-wave retarders, and the like. To facilitate explanation of theinvention, various examples of such retarders are provided below.

An optical retarder in accordance with one particular embodiment of theinvention will be described in connection with FIG. 1A. The opticalretarder 101 in FIG. 1A is formed of an acrylonitrile based polymericfilm. The film can be described by three mutually orthogonal axes,namely, two in-plane axes x and y and third axis z in the thicknessdirection of the film. As illustrated in FIG. 1B, the acrylonitrilebased retardation film 101 illustrated in FIG. 1A can be disposed on asubstrate 105. The substrate 105 may serve a variety of purposes. Forexample the substrate may be optically neutral, such as glass, and beused mainly for its mechanical properties and/or as a basis for affixingthe retardation film 101 to other optical elements. The substrate mayalso serve one or more optical functions. For example, the substrate maybe a mirror, a polarizer, or the like, where the retardation filmfunctions as an optical retarder in a larger optical construction. Thesubstrate may also be a polymeric film. The polymeric film may beisotropic or may be birefringent (in-plane or out-of-plane) to act incooperation with the optical performance of the acrylonitrile basedretarder to obtain a desired overall optical performance. The film mayalso be combined with a compensator film to improve optical performance.

Generally, the retardation film 101 can be used in connection with anysuitable substrate 105. The retardation film 101 may be laminated to thesubstrate, affixed with an adhesive, or otherwise suitably disposed onthe substrate. Care should be taken to ensure that the process andmanner used to dispose the retardation film 101 on the substrate 105does not adversely affect the optical performance of the ultimateretarder construction.

As described more fully below, it has been determined in connection withthe present invention that the acrylonitrile based retardation film isparticularly suited for use in applications where it is desirable tohave relatively uniform retardation of light incident on the retarderover a wide range of angles. Referring again to FIG. 1A, the retardationand angular performance of the retarder 101 are a function of thethickness d of the film and the relative refractive indices n_(x), n_(y)and n_(z) of the film for light polarized in the direction of the x, yand z axes, respectively. Birefringence along the in-plane axes, forexample, creates short and long paths for polarized light incident onthe film. Generally, the light is incident on the film with thepolarization direction aligned at an angle of 45° to the axes of thein-plane refractive indices.

The retardation of the film is defined generally as the phase differenceintroduced between the linear components of polarized light E_(p) andE_(s) aligned along directions parallel (p) and perpendicularly (s) tothe plane of incidence. In an ideal ¼-wave retarder, for example, lightpolarized along one axis (i.e., the component of the polarized lightalong the axis) is delayed, relative to light polarized along the otherin-plane axis, by one-quarter of its wavelength. When the polarizedlight is initially linearly polarized, the two components are eitherin-phase or 180° out-of-phase (i.e., the phase difference is equal to 0or π radians). When linearly polarized light passes through a ¼-waveretarder, a phase difference of π/2 radians is introduced between thetwo components. The total phase difference between the two componentsE_(p) and E_(s) is now π/2 or 3π/2 radians. In this manner, a ¼-waveretarder can be used to convert between linearly polarized light andcircularly polarized light.

When light is incident on the retarder at an angle normal to the planeof the retarder, the retardation is a function of the thickness of thefilm and difference in the in-plane refractive indices n_(y) and n_(x).As the angle of incidence deviates from normal incidence, theretardation of light passing through the retarder is also influenced bythe refractive index n_(z) for light polarized in the thicknessdirection z of the retarder. The off-normal performance of a givenretarder can be considered by comparing the magnitude of retardation atnormal incidence with the magnitude of retardation for incident lightwhich varies from normal incidence.

For a given retarder having known refractive indices, the relativemagnitude of retardation at different angles can be examined using therelationship $\begin{matrix}{{\delta = {d\left\lbrack {{\frac{n_{x}}{n_{z}}\left( {n_{z}^{2} - \left( {\sin \quad (\varphi)} \right)^{2}} \right)^{1/2}} - \left( {n_{y}^{2} - \left( {\sin (\varphi)} \right)^{2}} \right)^{1/2}} \right\rbrack}},} & (1)\end{matrix}$

where δ is the magnitude of retardation between the s and p fields, d isthe thickness of the film, n_(x), n_(y) and n_(z) are the respectiverefractive indices of the film for light of a given wavelength, and φ isthe angle of incidence in the x-z plane (measured from an axis normal tothe plane of the film). Thus, the magnitude of the retardation inequation (1) represents the difference in delay experienced by s- andp-polarized light components of the incident light, as the incidentlight passes through the retarder, as a function of incidence angle inthe x-z plane. It should be appreciated that equation (1) is provided asone way of expressing the retardation. A similar expression can bederived to express the retardation for light as a function of lightvarying in other planes (e.g., y-z plane).

In the above relationship, when light is incident on the film at adirection normal to the film (i.e., φ=0), the magnitude of retardation δreduces to a function of the thickness d and the in-plane refractiveindices difference described by the relationship

δ=d(n_(x)−n_(y)).  (2)

Thus, a desired retardation for light of normal angle incidence can beobtain by controlling the thickness of the film and the in-planerefractive indices. Higher retardation can be obtained by increasing thedifference between n_(x) and n_(y) and/or by increasing the thickness.

The amount of retardation desired depends generally on the particularapplication for which the retarder will be used and the wavelength oflight to be retarded. Typical ¼-wave retarders, for example, haveretardation values ranging from about 115 nm to about 158 nm. Typical½-wave retarders have retardation values ranging from about 230 nm toabout 316 nm. Full wave retarders could also be used to simply shift thephase of the two components by 2π radians. There are a number ofapplications particularly suited for acrylonitrile based retardershaving retardation values ranging from 115 nm to 158 nm. Unlessotherwise noted, in the discussion set forth below light having awavelength of about 550 nm (the approximate center wavelength of visiblelight) is used to characterize the performance of the retarder. Whilesuch light is appropriately used as a means for characterizing theretarder, it should be appreciated that the retarder may be used toretard light over the full visible range or at any particularwavelengths or wavelength bands thereof.

The difference between retardation of light incident at the normal angleand the retardation of light deviated from normal (off-normal) can beused to determine the appropriateness of the optical retarder for use inapplications where light is incidence on the retarder over a wide rangeof angles varying from normal to a maximum off-normal angle. Asdescribed more fully below, the acrylonitrile based retarder providesexceptional off-normal optical performance.

Generally, birefringence is induced in a polymeric material bystretching or drawing the material. As the material is stretched,molecules tend to align in the stretched direction. The inducedmolecular orientation creates refractive index differences for lightpolarized in the stretched and non-stretched directions. Stretchingpolymeric films not only induces a change in the refractive index forlight polarized in the stretch direction but may also induce changes inthe non-stretch and thickness directions. Under typical draw conditionsusing a tenter, for example, the changes in refractive indices for lightpolarized in the non-stretched and thickness direction are often quitedifferent. As a result, as a film is stretched to obtain a desiredin-plane refractive index mismatch, the thickness direction refractiveindex may not match either of the in-plane refractive indices. Whilesuch a change may not impact the performance of retarders used withnormal incident light, the change can significantly impact the retardersperformance when retarding off-normal incident light, especially whererelatively large angles are used.

From equation (1) it can be determined that improved off-normalperformance is obtained when the refractive index n_(z) for lightpolarized in the thickness direction is between the in-plane refractiveindices n_(y) and n_(x). Under typical drawing conditions, however, thethickness direction refractive index n_(z) of a drawn polymeric filmdoes not fall between the in-plane refractive indices n_(y) and n_(x).In accordance with the present invention, when acrylonitrile basedpolymers are stretched a desired mismatch can be obtained between thein-plane refractive indices while maintaining substantially equalrefractive indices for light polarized in the non-stretched andthickness directions. Moreover, closely matched n_(y) and n_(z)refractive indices are obtained even when the film is drawn in a mannerwhich constrains dimensional reduction in the non-stretched direction(e.g., when the film is stretched using a conventional tenter process).

As will be appreciated from equation (1), as the angle of incidenceincreases, the amount of retardation changes. In the acrylonitrile basedretarders of the present invention, the magnitude of the change inretardation is significantly reduced as a result of the substantiallyequal refractive indices (e.g., n_(y) and n_(z)). In contrast, a typicalbirefringent polymer such as polypropylene, for example, when stretchedin a conventional tenter exhibits a mismatch in the non-stretched andthickness direction refractive indices on the order of 0.009. As aresult of this mismatch, the off-normal performance of such a retarderis significantly impaired when compared to an acrylonitrile basedretarder.

While an acrylonitrile based retarding film can be used in retarderapplications using normal and near normal incident light, such aretarder is particularly suited for use in applications where theincident light varies from normal incidence to an off-normal angle ofincidence of at least about 30°. In such applications, using anacrylonitrile based retarder permits one to obtain a difference inretardation between the normal and off-normal incident light which isless than 15% of the normal incidence retardation, and is morepreferably less than 10% and even more preferably less than about 6%. Asthe off-normal angle of incidence increases the retardation differencealso increases. However, at off-normal angles as high as 60°, theretardation difference of the acrylonitrile based retarder is less than30% of the normal incidence retardation, and is more preferably lessthan about 25% and even more preferably less than about 20%. Anacrylonitrile based retarder may also be used to get uniform retardanceat lower angles of incidence. For example, advantages are obtained whenthe maximum off-normal angle is at least about 15° or even less.Exemplary embodiments of acrylonitrile based retarders are providedbelow.

As noted above, fabrication techniques suggested and used for theproduction of wide angle retarders are complex and expensive. Suchtechniques often involve steps of laminating multiple materialstogether, stretching birefringent materials using highly specializedequipment to artificially control the respective refractive indices, andthe like. In contrast, in one embodiment of the invention, anacrylonitrile based retarder can be fabricated using standard processingequipment, such as a tenter for stretching, with little or nomodification. Thus, significant cost savings can be achieved. Moreover,the process facilitates a high yield further reducing the costs ofproducing acrylonitrile based retarders.

One acrylonitrile based film found to be particularly suited for opticalretarders is a blend of an acrylonitrile phase and a toughening phase.An elastomeric (rubbery) copolymer, for example, may be used as atoughener in the blend. A number of advantages are obtained by theaddition of a toughening phase. For example, the resultant film willhave an increased resistance to impact and the film is rendered moreflexible and exhibits enhanced resistance to cracking, splitting andtearing. The elastomeric phase may also enhance the drawability.

The addition of a toughening phase, however, must also be taken intoaccount in the formation of the optical retarder. As described morefully below, in accordance with one embodiment an acrylonitrile basedpolymer and elastomeric copolymer blend is uniaxially stretched toobtain the desired birefringence and thickness of the retarder. Becausethe acrylonitrile polymer and the elastomeric copolymer are oppositelybirefringent in relation to the imposed strain, the strain-inducedchange in the refractive index of the elastomeric phase reduces theoverall retardation of the stretched film. Thus, blended acrylonitrilebased films including a elastomeric copolymer must be made thicker thanacrylonitrile based films without the elastomeric copolymer in order toobtain the same overall retardation. Increased thickness, however,increases overall absorption and off-normal retardation. These can leadto reduced transmitted intensities and/or off-normal color variations,both of which may be detrimental to many retardation applications.Increased thickness may also be desirable in certain instances toimprove film handling and processing (e.g., thicker film may be moreeasily laminated).

Depending on the application, different amounts of the tougheningcopolymer may be added to the blend. In general, a balance must bestruck between competing interests. For example, the amount of toughenerused must be weighed against the increased thickness required to obtaina desired retardation. Generally, where an elastomeric copolymer isused, it is desirable that the elastomeric phase be less than about18%-20% by weight. Where relatively high transmission is required it isdesirable that the elastomeric phase be less than about 15% and evenmore preferably less than about 10%, and still even more preferably,less than about 5%.

When using a toughener, other optical properties of the toughener mustalso be considered. It is generally desirable that the refractiveindices of the acrylonitrile based polymer and the toughener berelatively close. This is important to minimize diffuse scattering andreflection of the light passing through the retarder as it interactswith the different phases. In the above example, relatively closematching can be obtained by matching the isotropic refractive indicesprior to stretching of the acrylonitrile based polymer and theelastomeric copolymer. While this may not produce an exact match in thestretched film, due to different changes in the refractive indicesduring stretching, the indices are close enough for many applications.It also may be possible to select materials, composition and initialrefractive indices such that in the process of orientation therefractive indices of the two phases approach one another furtherreducing or eliminating hazing in the stretched film.

To facilitate an understanding of the present invention, exemplaryretarders comprised of an acrylonitrile based polymer/elastomericcopolymer blend will be described. While the examples below describe aprocess in which a web is cast and then oriented in the transversedirection using a tenter, any of a number of other typical filmprocessing techniques could be used. For example, the polymer may beextrusion-cast or solvent-cast. Webs may be cast on to an open-facedwheel or into a nip. Orientation may be affected in a variety of ways.For example, the film may be stretched uniaxially (machine or transversedirection) or biaxially using typical machine-direction orienters and/ortenters (e.g., mechanical and linear-motor). The film may also beoriented using a blown-film (e.g., single- and double-bubble) process,by calendering in a nip, by stretching the molten polymer into a webprior to cooling, and the like.

In one particular embodiment, the retarders are fabricated usingrubber-modified, acrylonitrile-methyl acrylate copolymers (72-99.5%copolymer, 18-0.5% elastomeric phase). The acrylonitrile-methyl acrylatecopolymer composition ranges from 70-100% acrylonitrile and 30-0% methylacrylate. The elastomeric phase contains from 70-90% butadiene with30-10% acrylonitrile. Rubber-modified, acrylonitrile-methyl acrylatecopolymers having 10% and 18% elastomeric phases are available from BPChemicals (Barex® 210 and 218).

While the example provided herein use acrylonitrile copolymerized withmethyl acrylate, other types of acrylonitrile based polymers may beused. For example, suitable copolymers containing acrylonitrile canobtained by copolymerizing acrylonitrile with a variety of(meth)acrylate monomers which have glass transition temperatures (Tg)which is less than about 20° C. Such (meth)acrylate monomers include,for example, methyl acrylate, propyl acrylate, butyl acrylate, isooctylacrylate, and 2-ethyl hexyl acrylate or a mixture of such monomers.

In accordance with one embodiment of the invention, rubber-modifiedacrylonitrile based optical retarders were fabricated. The retarderswere acrylonitrile based compositions having an elastomeric phase ofeither 10% or 18%. The copolymer phase composition was 75% acrylonitrileand 25% methyl acrylate. The elastomeric phase contained 70% butadieneand 30% acrylonitrile. As noted above, the inclusion of the elastomericphase provides toughness to the composition. The compositions of the twophases (copolymer and butadiene-based elastomeric phase) are selected toobtain closely matched refractive indices. Such compositions areavailable from BP Chemicals (Barex® 210 and 218) in extrusion andinjection molding grades. The Barex® family of acrylonitrile resins aretypically used to form high gas barrier packing materials and the like.

Webs of the above compositions were cast with an initial thicknessranging from 254-355 μm. The cast webs were processed to obtain targetretardation values of approximately 100-140 nm. The cast webs were drawnuniaxially in a tenter. Draw temperatures for such films generally rangefrom about 25° C. to 120° C. The draw temperatures are more preferablybetween about 90° C. and 110° C., and even more preferably between about90° C. and 105° C. Draw ratios for such a process range fromapproximately 1.5:1 to 5.0:1. The draw ratios are more preferablybetween about 2.0:1 and 5.0:1, and even more preferably between about2.5:1 and 4.0:1. Appropriate draw rates range from about 1% to 3000% persecond. The draw rates are more preferably between about 5% and 1000%per second , and even more preferably between about 10% and 200% persecond.

When using the 10% elastomeric composition, optical retardation filmsranging in thickness from 63-115 μm were produced which offered thetargeted retardation ranges. The films also exhibited minimal off-normalcoloration. The transmission intensity of such films exceeded 92%.

When using the 18% elastomeric compositions, it was apparent that thetarget retardation values could only be obtained with significantlythicker initial webs to increase the drawn thickness (e.g., 254-635 μm).Thus, retarders comprised of an 18% elastomeric phase exhibit reducedtransmission and worse off-normal performance. An optimum concentrationof the elastomeric phase appears to be between 5-10%. Suchconcentrations are believed to strike an optimum balance whererelatively high retardation values and light transmission were required.As described more fully below, a retarder manufactured from acomposition including about 10% of the elastomeric phase can befabricated in a relatively inexpensive manner and has relatively uniformperformance over a wide range of incidence angles.

As noted above, the inclusion of a toughener, such as an elastomericphase, in the acrylonitrile based composition tends to reduce theability to induce a desired birefringence in the film by stretching. Ina typical tenter process, because the film must be stretched in manyinstances near its breaking point to obtain the desired retardation, itis desirable that the initial web be substantially free of orientationin the machine direction prior to stretching. This is because initialorientation in the non-stretched direction must be overcome during thetenter operation before the desired orientation in the stretcheddirection can be obtained. Drawing 10% rubber-modified acrylonitrilebased webs in a tenter to obtain highly transmissive ¼-wave retarders,for example, typically requires that the pre-stretched webs be free fromany orientation in the machine direction. Thus, it is desirable that thecast webs must be initially cast in a manner which minimizes, or incertain instances eliminates, unintentional or residual molecularorientation in the machine direction.

In certain instances, the film may be drawn in the direction of aninitial orientation which relaxes the requirement for the castingprocess. For example, the film may be cast and then drawn in the machinedirection using a length orienter (LO). Such an LO process may takeadvantage of the initial machine direction. In fact, in such a case amachine direction orientation may be purposefully induced during castingto assist the formation of the desired birefringence. Other orientationprocesses could also be used. For example, machine direction orientationmay be induced in the molten polymer after it exits the die and prior tosolidification. In general, it is desirable that prior to stretching thefilm have no substantial orientation in the non-stretch direction,regardless of the manner and/or direction in which the film isstretched.

While cast webs of uniform thickness are described above, the thicknessof the cast web may also be altered. As noted above, retardation is afunction of the retarder thickness. Thus, retarder films having avarying retardation profile across the film may be produced bycontrolling the casting process to create thickness differences atdifferent points on the web.

The cast web of the present invention may be drawn in a directionorthogonal to the cast direction using a conventional tenter. The drawtemperature, rate and ratio are selected to induce a desired refractiveindex differential between in-plane refractive indices of the drawn web.In this manner, a desired retardation δ is obtained according to therelationship δ=d(n_(x)−n_(y)), while substantially matching therefractive index of the drawn web for light polarized in the non-drawnand thickness directions. The off-normal retardation can be determinedfrom Equation (1) (with n_(y) and n_(z) being substantially equal).

FIGS. 2A-2C illustrate various optical properties of acrylonitrile basedretarders in accordance with the present invention. Using the abovegeneral process, a transparent acrylonitrile based retarder film wasobtained. A 10% rubber-modified acrylonitrile-methyl acrylate 312 μmthick extruded web was used in which the initial isotropy wassubstantially preserved in its formation. The web was uniaxiallystretched to 3 times its original width in the cross direction. Thedrawing temperature was about 90° C. The resulting film wasapproximately 88.5 μm thick with refractive indices for 550 nm lightpolarized in the stretched direction of 1.5128 (n_(x)) and 1.5142 for550 nm light polarized in both the non-stretched and thicknessdirections (n_(y) and n_(z), respectively).

The retardation values for the above film were measured and comparedwith the retardation values determined using equation (1). FIG. 2Aillustrates a comparison of the measured retardation values 201 and theretardation values derived from the measured refractive indices usingequation (1) for the film at normal, 10°, 30° and 40° angles ofincidence. The difference in retardation as the angle of incidenceincreases from normal to 40 degree is approximately 10% (13 nm) ofnormal incidence retardation . In contrast, a polypropylene film havinga similar normal angle retardation will vary by approximately 50% (60nm) at a 40° angle of incidence. Retardation of a polystyrene film dropsby nearly 80% (100 nm) for light incident at 40° off-normal, whilehaving acceptable retardation performance at normal angle incidence.

Using equation (1), the retardation difference as the incident lightmoves from normal incidence for the above film was determined. FIG. 2Blists the retardation values (nm) of the film at different angles ofincidence 211, the change in retardation as the incident light movesoff-normal 213, and the retardation at the respective incident angles asa percentage of retardation at the normal angle incidence. FIG. 2Cillustrates a plot of the retardation values (nm) of the film as afunction of incidence angles.

A second acrylonitrile based retarder film was produced by uniaxiallystretching a 317.5 μm thick optically isotropic extruded film to 4 timesits original width in the cross direction at a temperature of 95° C. Theresulting film was approximately 84 μm. The refractive indices for lightpolarized along each direction, with n_(x) being the refractive indexfor light polarized in the stretched direction, were measured for lightat 488 nm, 550 nm and 700 nm as follows:

488 nm 550 nm 700 nm n_(x) 1.5162 1.5124 1.5055 n_(y) 1.5175 1.51391.5066 n_(z) 1.5174 1.5139 1.5066

FIG. 3A illustrate a Table which lists the retardation values (nm) 303for light of 550 nm incident on the film at various angles 301. FIG. 3Aalso lists the difference in retardation (nm) 305 as the incident lightmoves from normal incidence. FIG. 3A further lists the retardation atoff-normal angles of incidence as a percentage 307 of retardation valuesat normal angle incidence. FIG. 3B illustrates a plot of the retardationvalues (nm) 311 as a function of the angle of incidence. FIG. 3Cillustrates the retardation difference (nm) as a function of angle. Asillustrated in these FIGS., the off-normal performance of theacrylonitrile based retarder is relatively uniform compared to othersingle film retarders making the retarder well suited for a number ofapplications where uniform retardation is required for a wide range ofincidence angles.

As noted above, improved off-normal performance of theacrylonitrile-based retarder results from the matching of the n_(y) andn_(z) refractive indices. FIGS. 4A-4B illustrates how an increase in thedifference between n_(y) and n_(z) would impact the off-normalperformance of the retarder. In FIG. 4A, the retardation values 401 ofthe acrylonitrile-based retarder, as a function of incidence angle, areillustrated for the retarder described in connection with FIGS. 3A-3C.Columns 403, 405, and 407, illustrate the off-normal performance ofretarders having the same normal axis retardation values as thedifference in refractive indices n_(y) and n_(z) increases from 0.0003to 0.0009, respectively. The retardation at wide angles of incidencechanges significantly.

As FIG. 4A illustrates, larger differences between n_(y) and n_(z) causean increase in the drop in retardation at larger angles of incidence. Incertain applications, it may be desirable that the overall retardationdifference between normal incidence and incidence at 60° be less thanabout 20% (e.g., 20-30 nm) of the normal incidence retardation. This canbe obtained using an acrylonitrile based retarder which hassubstantially equal n_(y) and n_(z) refractive indices. For example, asthe above data illustrates, refractive indices equal to at least thefourth decimal places provide relatively uniform wide angle performance.FIG. 4B plots a comparison of the off-normal retardation 411 of theabove film with the those calculated from the refractive indices.

As illustrated by the data in FIGS. 4A and 4B, even slight variations inn_(y) and n_(z) can significantly impact the off-normal performance ofthe retarder. This reinforces the particular suitability of theacrylonitrile-based optical retarders, particularly such retarders usedin applications where uniform, wide-angle performance is desired.Moreover, such retarders can be fabricated using a process which permitsproduction of relatively large retarders having uniform thickness andoptical characteristics and which is relatively simple.

While in the above examples, a butadiene elastomeric toughening materialis added to the acrylonitrile-based retarder, it will be appreciatedthat other acrylonitrile-based retarders will have similar desirableoptical properties. In general, other suitable materials may be added tothe retarder, so long as the materials do not significantly impact theoptical performance of the retarder. For example, isoprene basedrubbers, natural rubbers and the like could be used.

As noted above, acrylonitrile based retarders are particularly suitedfor applications requiring relatively uniform retardation over a widerange of incidence angles. More particular embodiments of the inventionare described below in such applications.

In accordance with one embodiment of the invention, the acrylonitrilebased retarder is used as the basis of a retarder or polarizationrotating mirror. By way of example, without intending to be limited tothe example, a particular ¼-wave mirror will be described. The ¼-wavemirror is used to rotate the polarization direction of linearly polarizelight, reflected from the mirror, by 90°. One particular ¼-wave mirrorarrangement 500 is illustrated in FIG. 5A. An acrylonitrile basedretarder 501 is disposed in a plane parallel to the reflecting surfaceof a mirror 503. A light source directs linearly polarized light 505 tothe mirror at an incidence angle φ. The retarder 501 is orientedrelative to the incident light such that light incident on the retarderat an angle normal to its surface (i.e., φ=0) is retarded by one quarterof its wavelength. In this construction, the linearly polarized light505 is converted to circularly polarized light 505A, with a firstrotation direction, as it passes through the retarder 501.

The circularly polarized light 505A reflects off the surface of themirror 503. Light 505B reflected by mirror 503 is circularly polarizedwith an opposite rotation direction. The reflected light is directedback onto retarder 501 at an angle φ. As the reflected circularlypolarized light 505B passes through the retarder 501 a second time,another ¼-wave phase difference is introduced converting the circularlypolarized light 505B into linearly polarized light 507. The polarizationdirection of the reflected linearly polarized light 507 is substantiallyorthogonal to the initial polarization direction of the incident light505.

As will be appreciated, the above-description assumes normal incidenceand precise ¼-wave delays for each pass through the retarder 501. As theangle of incidence φ varies from normal, the relative phase shift willbe impacted as a result of off-normal retardation deviations of theretarder 501. Thus, as the linearly polarized light 505 passes throughthe retarder at higher angles of incidence, the ellipticity introducedinto the polarized light by the retarder 501 increases. As off-normalincident light is reflected, it also passes back through the retarder atan incidence angle φ, assuming a substantially flat mirror. Theellipticity introduced by the off-normal retardation difference will addto the ellipticity introduced by the first pass.

As the above discussion illustrates, the initial linearly polarizedlight 505 passes through the retarder twice. An ellipticity introducedinto the polarized light 507 reflected from the ¼-wave mirror 500 willvary with the angle of incidence. Such ellipticity tends to degradeperformance of those applications which rely on the linear polarizationstate of the reflected light. Accordingly, in applications using ¼-wavemirrors and relatively wide angles of incidence, it is desirable tominimize the off-normal retardation difference so that the reflectedlight will be substantially linearly polarized, with the direction ofpolarization being rotated by 90°.

As will be appreciated from the above description, a ¼-wave mirror 500constructed with an acrylonitrile based retarder provides relativelyuniform off-normal performance in a form which can be constructed atrelatively low cost and complexity. The construction allows rotation ofthe polarization direction of linearly polarized light withoutintroducing substantial ellipiticity to the rotated linearly polarizedlight at relatively high angles of incidence. In general, it isdesirable that any deviation from an ellipticity of 0 introduced atoff-normal angles of incidence, be less than about 10%. It is morepreferable that the deviations be less than about 5%. In certainincidences, it is necessary that the ellipiticity be less than 1% forall angles of incidence. As will be appreciated from the abovedescription of the acrylonitrile based retarder, the above results canbe obtained due to the particular wide-angle optical performance of theretarder.

In FIG. 5A, the retarder 501 is illustrated as being separate from themirror 503. In FIG. 5B, another embodiment of retarding mirror 510 isillustrated in which an acrylonitrile based retarder 511 is laminated orotherwise affixed to a mirror 513 by an adhesive 514. The opticalperformance of the mirror arrangement is generally the same as thatdescribed above in connection with FIG. 5A. Consideration, however,should be given to any additional components introduced by theconstruction. For example, lamination defects, refractive indices ofadhesives, and the like must be considered.

An optical system incorporating a retarder mirror, of the typeillustrated in FIGS. 5A and 5B, for example, is illustrated in FIG. 6,The optical system of FIG. 6 is a projection display system 600incorporating a folded light path. The general operation of the foldedpath projection display system is illustrated in FIG. 6. As will bedescribed more fully below, the projection system 600 incorporates anacrylonitrile based ¼-wave retarder/mirror arrangement 605 as a keyelement which must operate over a large range of incidence angles. Theoperation of the display system 600 also requires that light reflectedby the ¼-wave retarder/mirror arrangement 605 be highly linearlypolarized (e.g., exhibit minimal ellipticity). For a more detaileddescription of such systems, reference may be made to U.S. Pat. No.5,557,343, entitled Optical System Including a Reflective Polarizer fora Rear Projection Picture Display Apparatus, and Published EuropeanApplication EP 0,783,133 entitled Projecting Images.

In the optical system of FIG. 6, light, representative of an image to bedisplayed, is projected from an image source 601 onto a screen assembly603. The light 602 from the source 601 is linearly polarized in a firstdirection. The rear surface of the screen assembly 603 includes areflective polarizer. Reflective polarizing films are available fromMinnesota Mining and Manufacturing Company under the name of the DualBrightness Enhancement Film (DBEF). Other reflective polarizing filmsare described in U.S. patent applications Ser. No. 08/402,041, filedMar. 10, 1995 and entitled Optical Film, and Ser. No. 08/610,092, filedFeb. 29, 1996, entitled An Optical Film, the contents of which areincorporated herein by reference.

The reflective polarizer of the screen assembly reflects light of oneparticular linear polarization and transmits light of an opposite(orthogonal) linear polarization. The orientation of the reflectivepolarizer and the polarization direction of light initially incident onthe reflective polarizer are such that the incident light is initiallyreflected by the reflective polarizer toward the acrylonitrile basedretarding mirror assembly 605. The retarding mirror assembly 605 may beof the type illustrated in FIGS. 5A and 5B and serves to rotate thepolarization direction of the linearly polarized light by 90°.

In operation, linearly polarized light reflects from the reflectivepolarizer and is incident on the retarding mirror 605. The light isreflected and the polarization direction is rotated by 90° such that thepolarization direction now aligns with the pass direction of thereflective polarizer. Thus, the light passes through the screen assembly603 for viewing. It is desirable that all of the light pass through thescreen to increase viewing brightness. Any ellipticity in the light,however, will reduce the amount of light passing through the screensince the component of light still aligned in the direction of theoriginal polarization will be reflected by the screen.

As will be appreciated from the optical geometry illustrated in FIG. 6,light will be incident upon the retarder mirror 605 over a large rangeof incidence angles φ₁, φ₂, . . . φ_(n). In such an application, themaximum angle of incidence may be quite high. As noted above, anyellipticity introduced into the light reflected from the retarder mirror605 will degrade the overall performance of the display device. In theprojecting device illustrated in FIG. 6, the retarder mirror 605 isformed of an acrylonitrile based retarder so as to minimizes theellipiticity introduced into the reflected light as angles of incidencevary. It is generally desirable, that deviations from an ellipiticity ofzero in such a system be less than 5%. It is more preferable, in certaininstances to have the ellipiticity be even less than 1%. While the abovediscussion assumes an ellipticity of zero at normal angels of incidence,the preferred percentages for maximum ellipticity are appropriate iflight incident on the retarder mirror at normal angles of incidence alsoreflects from the retarder mirror with some ellipticity.

The acrylonitrile based retarders described above can be incorporatedinto a retarding mirror exhibiting ellipticity variation within theabove tolerances. Thus, the projection device, incorporating anacrylonitrile based retarder mirror, will have improved performance overmany typical retarders and can be manufactured relatively inexpensively.Moreover, an acrylonitrile retarder manufactured as described above, iswell suited for lamination to mirror surfaces and other substrates.

FIG. 7 illustrates an optical construction in accordance with stillanother particular embodiment of the invention. In the embodiment ofFIG. 7, an acrylonitrile based retarder 701 is incorporated into ananti-glare optical construction. The anti-glare optical constructionincludes an absorptive polarizer 703, such as a dichroic polarizer, forexample. The polarizer 703 linearly polarizes unpolarized light 705incident on the polarizer. The acrylonitrile based retarder 701 isoriented relative to the absorbtive polarizer to convert the linearlypolarized light 706 to circularly polarized light having a firstrotation direction. If the circularly polarized light is reflected offthe surface of an optical element 707, which is protected by theanti-glare construction, the light is reflected as circularly polarizedlight rotating in the opposite direction. The circularly polarized light708 passes back through the retarder 701. Thus, as in the case of theretarding mirror above, the polarization direction of the light is nowrotated by 90°. The light, rotated by 90°, strikes the absorptivepolarizer, this time linearly polarized in a direction of theabsorption, to thereby inhibit or prevent light reflected from thesurface of the optical element 707 from exiting the antiglareconstruction.

The optical element 707 may be any type of reflective surface where itis desirable to reduce glare. For example, it may be the screen of acomputer monitor. In such an application, the polarizer 703 andacrylonitrile based retarder 701 can be affixed or otherwise positionedin front of the monitor in any of a variety of ways conventionallyknown. As will be appreciated, when the optical element 707 is amonitor, light 721 exiting the monitor will pass through theacrylonitrile based retarder 701 and will be polarized by the absorptivepolarizer 703.

As in the above description of the retarder mirror, the off-normalperformance of the acrylonitrile based retarder is important to ensurethat light which is reflected from the optical element 707 is correctlypolarized to be absorbed by the absorptive polarizer 703 uponreflection. Further, it will be appreciated that relatively large anglesof incidence φ may be seen by the anti-reflective optical construction.For example, computer monitors are often used in environments wherelight sources causing reflection and glare are positioned at an anglerelatively oblique with respect to the monitor. Thus, improvedwide-angle performance of the retarder, serves to further reduce oreliminate the glare or reflection from the optical element 707 beingprotected.

In one embodiment, the surface of the absorptive polarizer 703 facingthe incident light may be A/R coated to reduce any reflection from thepolarizer. The absorptive polarizer 703 may also be laminated orotherwise affixed to a substrate such as glass or other films. Thesubstrate may also be A/R coated. The ¼-wave film 701 may also beaffixed by lamination or otherwise to a substrate. In certain instancesthis may be the same substrate to which the polarizer is affixed. Thevarious elements may be laminated between glass. One or more of theglass surfaces may also be A/R coated.

As noted above, the present invention is applicable to a number ofoptical retarders. It is believed to be particularly useful inapplications where light is incident on the retarder over a wide rangeof angles. Accordingly, the present invention should not be consideredlimited to the particular examples described above, but rather should beunderstood to cover all aspects of the invention as fairly set out inthe attached claims.

What is claimed is:
 1. An anti-reflective (AR) optical element operable when disposed proximate to a partially reflective surface to substantially reduce reflection of visible light from the surface over a relatively wide range of incidence angles, the AR optical element comprising: an absorptive polarizer having an incident light side and a reflective surface side; and an acrylonitrile-based optical retarder disposed adjacent the reflective surface side of the absorptive polarizer, wherein a retardation magnitude for light at of a wavelength of interest incident on the retarder at an angle normal to a surface of the retarder is within 15% of a retardation magnitude for light of the wavelength of interest incident on the retarder at an angle of incidence of at least about 30°, measured from normal incidence.
 2. An anti-reflective (AR) optical element as recited in claim 1, wherein the acrylonitrile-based optical retarder comprises a blend of an acrylonitrile based polymer and an elastomeric copolymer.
 3. An anti-reflective (AR) optical element as recited in claim 1, further comprising a substrate having a first and second side, the first side of the substrate being affixed to the incident light side of the absorptive polarizer, wherein an antireflective coating is coated on the second side of the substrate.
 4. An anti-reflective (AR) optical element as recited in claim 1, wherein the AR optical element is adapted to reduce glare reflections from a screen of a computer monitor.
 5. An optical element operable over a relatively wide range of incidence angles, comprising: an absorptive polarizer; and an acrylonitrile-based optical retarder disposed adjacent a side of the absorptive polarizer, wherein a retardation magnitude for light at of a wavelength of interest incident on the retarder at an angle normal to a surface of the retarder is within 15% of a retardation magnitude for light of the wavelength of interest incident on the retarder at an angle of incidence of at least about 30°, measured from normal incidence. 